Radio communication systems are used in many different applications. For example, law enforcement and emergency personnel frequently use radio systems to communicate with each other. In some applications, simulcast radio communication systems are used to communicate with a number of fixed or mobile radios (hereafter called “terminals”) spaced over a large geographic area. Simulcast communication systems operate by transmitting from multiple site locations, hereafter called “sites.” Each site has one or more base stations that transmit a signal to communicate with the terminals. In a simulcast system, communication signals typically are transmitted at about the same time by all sites in the system. The communications may be received by a variety of different types of terminals, including handheld radios, vehicular radios, etc. A given radio will receive communications from the system as long as the radio is within the transmission range of at least one of the multiple simulcast sites.
In some situations, a radio may experience interference if it is within the transmission range of more than one simulcast site. For example, if a terminal is within range of two simulcast sites, it will receive the same communication signal from both sites—but not necessarily at the same time. This typically occurs if the terminal is closer to one site that it is to the other site. In this case, because of the different transmission distances, there will be a slight delay in receiving the signal from the further site (relative to receipt of the same signal from the nearer site). As a result of this delay, the signals received from the two sites may interfere with one another. This interference is a form of “delay spread fading.”
High delay spreads in a simulcast system can result in significant signal degradation, with the degree of degradation influenced by the type of modulation that is used. For digital modulations, a metric for quantifying delay spread degradation is the bit error rate (BER).
One type of modulation is defined by the Project 25 (P25) standard for public safety radio communications. Project 25 is defined by a suite of American National Standards and other documents developed by the Telecommunications Industry Association (TIA). The P25 Phase 1 specification includes two alternative digital modulation schemes, C4FM and CQPSK. C4FM is a constant-envelope, four-level frequency modulation scheme that operates in 12.5 kHz channels. CQPSK is a compatible differential four-level quadrature phase shift keying modulation scheme, which, when designed in accordance with P25 specification design parameters, requires less bandwidth than C4FM. Both C4FM and CQPSK as defined in the P25 specification provide a symbol transmission rate of 4800 baud using two bits per symbol. The resulting total channel throughput is 9600 bits per second (bps).
The amount of delay spread degradation in a simulcast system is influenced by the duration of the delay relative to the symbol transmission period. The C4FM and CQPSK modulation schemes used in P25-compatible systems transmit one symbol every 208.33 microseconds. When the delay spread in these systems is less than 20 microseconds, the resulting bit error rate for strong signals typically is less than 1%. As the delay spread increases, however, the bit error rate also increases. For example, a delay spread of 60 microseconds results in a strong-signal bit error rate of over 5%, which is enough to substantially degrade communication quality. With higher delay spreads, the degradation in communication quality becomes even worse.
Simulcast systems typically are designed to minimize delay spread. To a certain extent, system designers are able to reduce delay spread by adjusting site placement, antenna gain, tower height, and signal transmission timing. As a practical matter, however, it is extremely difficult if not impossible to entirely eliminate delay spread, and relatively high delay spreads (i.e., 100 microseconds or more) are not uncommon.
Accordingly, there is a need for systems and methods that provide for simulcast transmission with reduced bit error rates. There also is a need for systems and methods with reduced bit error rates that are compatible with existing simulcast systems, such as P25-compatible systems.